An exemplary traveling wave tube (TWT) 20 is illustrated in FIG. 1. The elements of TWT 20 are generally coaxially arranged along a TWT axis 22. They include an electron gun 24, a slow wave structure (SWS) 26 (embodiments of which are shown in FIGS. 2a and 2b), a beam focusing arrangement 28 which surrounds SWS 26, a microwave signal input port 30 and a microwave signal output port 32 which are coupled to opposite ends of SWS 26, and a collector 34. A housing 36 is typically provided to protect the TWT elements.
In operation, a beam of electrons is launched from electron gun 24 into SWS 26 and is guided through the SWS by beam focusing arrangement 28. A microwave input signal 38 is inserted at input port 30 and moves along SWS 26 to output port 32. SWS 26 causes the phase velocity (i.e., the axial velocity of the signal's phase front) of the microwave signal to approximate the velocity of the electron beam.
As a result, the beam's electrons are velocity-modulated into bunches which overtake and interact with the slower microwave signal. In this process, kinetic energy is transferred from the electrons to the microwave signal; the signal is amplified and is coupled from output port 32 as an amplified microwave output signal 40. After their passage through SWS 26, the beam's electrons are collected in collector 34.
Beam focusing arrangement 28 is typically configured to develop an axial magnetic field. A first configuration includes a series of annular, coaxially arranged permanent magnets 42 which are separated by pole pieces 44. Magnets 42 are typically arranged so that adjacent magnet faces have the same magnetic polarity. This beam focusing arrangement is comparatively light weight and is generally referred to as a periodic permanent magnet (PPM). In TWTs in which output power is more important than size and weight, a second beam focusing configuration often replaces the PPM with a solenoid 46 (partially shown adjacent input port 30) which carries a current supplied by a solenoid power supply (not shown).
As shown in FIGS. 2a and 2b, SWSs generally receive an electron beam 48 from electron gun 24 into an axially repetitive structure. A first exemplary SWS is helix member 50 shown in FIG. 2a. A second exemplary SWS is coupled cavity circuit 52 shown in FIG. 2b. Coupled cavity circuit 52 includes annular webs 54 which are axially spaced to form cavities 56. Each one of webs 54 forms a coupling hole 58 which couples a pair of adjacent cavities. Helix member 50 is especially suited for broadband applications while coupled cavity circuit 52 is especially suited for high power applications.
TWTs are capable of amplifying and generating microwave signals over a considerable frequency range (e.g., 1-90 GHz). They can generate high output powers (e.g., &gt;10 megawatts) and achieve large signal gains (e.g., 60 dB) over broad bandwidths (e.g., &gt;10%).
Electron gun 24, helix member 50 (with input port 30 and output port 32) and collector 34 of TWT 20 illustrated in FIG. 1 are again shown in the TWT schematic of FIG. 3 (for clarity of illustration, SWS 26 is not shown in the schematic). Electron gun 24 has a cathode 60 and an anode 62. Collector 34 has a first annular stage 64, a second annular stage 66 and a third stage 68. Because third stage 68 generally has a cup-like or bucket-like form, it is sometimes referred to as the "bucket" or "bucket stage."
Helix member 50 and body 70 of TWT 20 are at ground potential. Cathode 60 is biased negatively by a voltage V.sub.cath from a cathode power supply 72. An anode power supply 74 is referenced to cathode 60 and applies a positive voltage + to anode 62. This positive voltage establishes an acceleration region 76 between cathode 60 and anode 62. Electrons are emitted by cathode 60 and accelerated across acceleration region 76 to form electron beam 48.
As described above with reference to FIG. 1, electron beam 48 travels through helix member 50 and exchanges energy with a microwave signal which travels along the helix member from input port 30 to output port 32. Only a portion of the kinetic energy of electron beam 48 is transferred in this energy exchange. Most of the kinetic energy remains in electron beam 48 as it enters collector 34. A significant part of this kinetic energy can be recovered by decelerating the electrons before they are collected at the collector walls.
Because of their negative charge, the electrons of electron beam 48 form a negative "space charge" which would radially disperse the electron beam in the absence of any external restraint. Accordingly, beam focusing arrangement 28 (see FIG. 1) applies an axially directed magnetic field which restrains the radial divergence of electrons by causing them to spiral about electron beam 48.
However, electron beam 48 is no longer under this restraint when it enters collector 34 and, consequently, it begins to radially disperse. In addition, the interaction between electron beam 48 and the microwave signal on helix member 50 causes the beam's electrons to have a "velocity spread" as they enter collector 34 i.e., the electrons have a range of velocities and kinetic energies.
Electron deceleration is achieved by application of negative voltages to collector 34. The potential of collector 34 is "depressed" from that of TWT body 70 (i.e., made negative relative to the TWT body). The kinetic energy recovery is further enhanced by using a multistage collector, e.g., collector 34, in which each successive stage is further depressed from the body potential of V.sub.B. For example, if first collector stage 64 has a potential V.sub.1, second collector stage 66 has a potential V.sub.2 and third collector stage 68 has a potential of V.sub.3, these potentials are typically related by the equation V.sub.B =0&gt;V.sub.1 &gt;V.sub.2 &gt;V.sub.3, as indicated in FIG. 3.
The voltage v.sub.1 on first stage 64 is depressed sufficiently to decelerate the slowest electrons 80 in electron beam 48 and yet still collect them. If this voltage V.sub.1 is depressed too far, first stage 64 repels rather than collects electrons 80. These repelled electrons may flow to TWT body 70 and reduce the TWT's efficiency. Alternatively, they may reenter the energy exchange area of helix member 50 and reduce the TWT's stability.
Similar to first stage 64, successively depressed voltages are applied to successive collector stages to decelerate (but still collect) successively faster electrons in electron beam 48, e.g., electrons 82 are collected by collector stage 66 and electrons 84 are collected by collector stage 68.
In operation, the diverging low kinetic energy electrons 80 are repelled by collector stage 66, which causes their divergent path to be modified so that they are collected on the interior face of the less depressed collector stage 64. Higher energy electrons 82 are repelled by collector stage 68, which causes their divergent paths to be modified so that they are collected on the interior face of the less depressed collector stage 66. Finally, the highest energy electrons 84 are decelerated and collected by collector stage 68. This process of improving TWT efficiency by decelerating and collecting successively faster electrons with successively greater depression on successive collector stages is generally referred to as "velocity sorting."
The efficiency gain realized by velocity sorting of electron beam 48 can be further understood with reference to current flows through collector power supply 86 which is coupled between cathode 60 and collector stages 64, 66, and 68. If the potential of collector 34 were the same as TWT body 70, the total collector electron current I.sub.coll would flow back to cathode power supply 72 as indicated by current 88 in FIG. 3, and the input power to TWT 20 would substantially be the product of the cathode voltage V.sub.cath and the collector current I.sub.coll.
In contrast, the currents of multistage collector 34 flow through collector power supply 86. The input power associated with each collector stage is the product of that stage's current and its associated voltage in collector power supply 86. Because the voltages V.sub.1, V.sub.2, and V.sub.3 of collector power supply 86 are a fraction (e.g., in the range of 30%-70%) of the voltage of cathode power supply 72, the TWT input power is effectively decreased.
Efficiencies of TWTs with multistage collectors are typically in the range of 25%-60%, with higher efficiency generally associated with narrower bandwidth. These efficiencies can be further improved by enhancing the velocity sorting of the collector and considerable efforts have been expended toward this goal in the areas of collector design, simulation and prototype testing.
In some collectors, velocity sorting is improved by configuring a collector stage to introduce transverse asymmetries of the electric field within that stage. These transverse asymmetries can often enhance velocity sorting by selectively moving electrons away from the electron beam's axis.
For example, some of the low kinetic energy electrons 80 in FIG. 3 may travel along the collector axis (generally, axis 22 of FIG. 1). When these electrons are repelled by the higher depressed collector stages, they may reverse their path and travel back along the collector axis into the energy exchange area of the helix member 50. A transverse asymmetry in the electric field causes these electrons to diverge from the collector axis and increase the probability that they will be collected by the collector stage 64.
Transverse field asymmetries (electric or magnetic) are conventionally realized, for example, by beveling the leading edge of first collector stage's aperture 92 or by attaching external magnets to the collector body. Although these structures can improve velocity sorting, the former cannot be easily modified and the latter is expensive, time-consuming and adds weight and parts complexity.
Because the efficiency of a collector is a function of many elements, (e.g., diameter, length and shape of each stage, spatial interrelationship of stages, stage materials and interaction variations in the SWS), even complex computer modeling does not completely predict a design's performance.
Even well-designed velocity sorting may be degraded by the introduction of unexpected asymmetries, e.g., by manufacturing tolerances. Consequently, extensive and expensive prototype testing and design modification are often required to finalize a collector design and time-consuming test adjustments (e.g., attachment of external magnets) are often required during production because of the lack of any ready means for adjusting transverse electric field distributions within a collector.